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Full text of "The manufacture and properties of iron and steel"

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CHAPTER XIV.
INFLUENCE OP HOT WORKING ON STEEL.
SECTION XlVa.—Effect of thickness upon the physical prop-' erties.—One of the fundamental difficulties in writing specifications is to decide the nature of the test-piece to be required, inasmuch as the strength and ductility will vary in pieces of different thickness, while the results will not be alike in tests cut from different structural shapes, like plates, angles and rounds, even though they be rolled from the same steel. From one point of view each piece of metal throughout a bridge should be of exactly the same strength per unit of section without regard to its thickness; but in taking this as a basis a serious trouble is encountered. Suppose, for instance, that a metal is required running between 56,000 and 64,000 pounds per square inch, and a charge is made which in three-, eighth-inch plate gives 57,000 pounds'. If this steel be rolled into seven^eighth-inch angles, or into one-inch plate, or into two-inch rounds, it is quite probable.that these will run below the allowable minimum. On the other hand, if- the steel gives 62,000 pounds in a preliminary test, the larger sections will give proper results, while one-quarter-inch plate wilLbe too high in ultimate strength.
Where a structure is to be made of large quantities of very large or. very, small-sections, it" is well to specify that the test shall be' made on the special thicknesses needed, but. in ordinary cases itv seems absurd to the .practical mind that a,heat of steel can be perfectly suitable for .one size and unsuitable for another. It was the .custom in the past for inspectors to recognize the situation and make tests from the usual sizes, with a full knowledge that thicker . and thinner, members would give different results, but in later practice there is a growing tendency to test each separate thickness, a change which has been the cause of great expense to the manufacturer. Provisions to cover this point should be incorporated into contracts and a certain definite allowance made for variations in the dimensions of ,the finished material. On :the other hand the i
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